1. Field of the Invention
This invention concerns an organic electroluminescence device capable of taking out emission of light on the side of a cathode of the device.
2. Description of Related Art
Electroluminescence devices utilizing electroluminescence (hereinafter simply referred to as an EL device) have been noted for the use of them as light emitting devices in various kinds of display devices since they have features of high visibility due to self-emission and excellent impact resistance being a completely solid devices.
The EL devices include inorganic EL devices using inorganic compounds as the light emitting material and organic EL devices using organic compounds as the light emitting material. Among them, since the organic EL devices can be easily reduced in the size with a driving voltage lowered remarkably studies intending for their practical use have been made earnestly as display devices in next generation. The organic EL device basically adopts a structure comprising a lamination of anode/light emitting layer/cathode in which a transparent electrode formed on a substrate using a glass plate or the like (usually adopt). In this case, emission light is taken out on the side of the substrate.
By the way, it has been attempted to take out emission light from the side of the cathode by making the cathode transparent because of the following reasons in recent years. At first, when the anode is made transparent together with the cathode, a light emission device transparent as a whole can be provided. An optional color can be adopted as the background color for the transparent light emission device, and a display which is colorful in a state other than light emission can be obtained, to enhance the decorative property. When black is adopted for the background color, contrast upon emission is improved. Next, when a color filter or a color conversion device is used, such component can be placed on the light emission device. Accordingly, the device can be manufactured with no particular consideration on these layers. As the merit of this, for example, a substrate temperature can be made higher upon forming the anode thereby enabling to lower the resistance value of the anode.
Since the foregoing advantages can be obtained by making the cathode transparent, it has been attempted to manufacture an organic EL device by using a transparent cathode. For example, Japanese Patent Laid-Open Hei 10-162959 disclose an organic EL device in which an organic layer containing an organic light emitting layer is interposed between an anode and a cathode, and the cathode comprises an electron injecting metal layer and an amorphous transparent conductive layer, and the electron injecting metal layer is in compact with the organic layer. For illustrating the background of the invention, constitutions described above will be explained briefly.
At first, the amorphous transparent conductive layer constituting the cathode in the organic EL device is to be explained. Any of amorphous transparent conductive layers may be used so long as it is amorphous and has transparency and it preferably comprises a specific resistivity of 5xc3x9710xe2x88x924 xcexa9xc2x7cm or lower for avoiding voltage drop and not uniform light emission attributable to this. Further, Inxe2x80x94Znxe2x80x94O series oxide films are preferred for the material. Inxe2x80x94Znxe2x80x94O series oxide layer is a transparent conductive film comprising an amorphous oxide containing indium (In) and zinc (Zn) as main cationic elements.
Then, the electron injecting metal layer is to be explained. The electron injecting metal layer is a layer of a metal capable of satisfactorily injecting electrons to an organic layer containing a light emitting layer. It is desirable that the light transmittance is 50% or higher for obtaining a transparent light emitting device and, for this purpose, an ultra thin film of about 0.5 to 20 nm thickness is desirable. The electron injecting metal layer can include, for example, those layers of 1 nm to 20 nm film thickness by using metals having a work function of 3.8 eV or lower (electron injecting metal), for example, Mg, Ca, Ba, Sr, Li, Yb, Eu, Y and Sc. A constitution providing light transmittance of 50% or more, particularly, 60% or more is preferred in this case.
The organic layer interposed between the anode and the cathode contains at least a light emitting layer. The organic layer may be a layer only consisting of a light emitting layer, or it may be a multi-layered structure in which a hole injecting/transporting layer or the like is laminated together with the light emitting layer. In the organic EL device, the organic layer has (1) a function capable of injecting holes by the anode or the hole transport layer and capable of injecting electrons from the electron injecting layer, (2) a transporting function of moving the injected charges (electrons and holes) under the effect of an electric field and a light emitting function of providing re-combination sites of electrons and holes in the inside of the light emitting layer and providing light emission therefrom. The hole injecting/transporting layer is a layer comprising a hole transfer compound, which has a function of transmitting holes injected from the anode to the light emitting layer, in which more holes are injected to the light emitting layer at a lower electric field by interposing the hole injecting/transporting layer between the anode and the light emitting layer. In addition, electrons injected from the electron injecting layer in the light emitting layer are accumulated near the boundary in the light emitting layer by the energy barrier present at the boundary between the light emitting layer and the hole injecting/transporting layer thereby improving the emission efficiency of the EL device and providing an EL device of excellent emitting performance.
There is no particular restriction on the anode so long as the anode shows conductivity of the work function of 0.8 eV or higher. A metal, a transparent conductive film (conductive oxide film) or combination of them having a work function of 4.8 eV or higher is preferred. It is not always necessary that the anode is transparent and, for example, a carbon layer of black color may be coated. Suitable metal can include, for example, Au, Pt, Ni and Pd and suitable conductive oxide can include, for example, Inxe2x80x94Znxe2x80x94O, Inxe2x80x94Snxe2x80x94O, ZnOxe2x80x94Al and Znxe2x80x94Snxe2x80x94O. Further, the laminate can include, for example, a laminate of Au and Inxe2x80x94Znxe2x80x94O, a laminate of Pt and an Inxe2x80x94Znxe2x80x94O and a laminate of an Inxe2x80x94Snxe2x80x94O and Pt. Since it may suffice that the boundary with the organic layer in the cathode has a work function of 4.8 eV or higher, the anode may be formed as a dual layer in which a conductive film of a work function of 4.8 eV or lower may be used on the side not in contact with the organic layer. In this case, a metal such as Al, Ta or W or an alloy such as Al alloy or Taxe2x80x94W alloy may also be used. Furthermore, a conductive polymers such as doped polyaniline or doped polyphenylenevinylene, an amorphous semiconductor such as xcex1-Si, xcex1-SiC or xcex1-C or crystallite such as a xcexcC-Si or xcexcC-SiC may also be used. Furthermore, Cr2O3, Pr2O5, NiO, Mn2O5 or MnO2 as black semiconductive oxide may also be used.
As has been described above, Japanese Patent Laid-Open Hei 10-162959 discloses a technique of taking out light from the side of the cathode by forming the cathode with an ultra-thin electron injecting metal layer and an amorphous transparent conductive layer. However, improvement for the anode is not mentioned. That is, the literature contains no descriptions for the anode on the lower side which is effective for efficiently taking out light from the cathode on the upper side. It merely describes that the use of a metal, a transparent conductive film or a combination of them with conductivity showing a work function of 4.8 eV or higher can be used for the anode. It mentions to Au, Pt, Ni and Pd as suitable metals. However, such metals can not be said to have satisfactory adhesion with the organic layer but tend to cause dark spots (non-emission point) or not uniform light emission. Further, fine fabrication technique for the metal has not yet been established and highly fine pattering is difficult.
In view of the technical subjects in the prior art described above, this invention intends to provide an organic electroluminescence device having an anode constitution on the lower side which is effective to efficiently take out light from the cathode on the upper side.
The purpose of the invention can be attained in accordance with this invention in an organic electroluminescence device comprising an anode, a cathode and an organic layer put between them, in which the organic layer contains an organic light emitting layer that emits light by re-combination of holes supplied from the anode and electrons supplied from the cathode, wherein
the anode contains a metal belonging to the group V or group VI of the periodical table to at least a portion in contact with the organic layer.
Preferably, the metal is selected from chromium, molybdenum, tungsten, tantalum and niobium,
The work function of the metal is 4.8 eV or lower. Further, the anode has a reflectance of 40% or higher.
The anode is light reflecting while the cathode is light permeable and emission light is mainly emitted from the side of the cathode.
Preferably, the cathode, the organic layer and the anode are laminated orderly from above to a substrate.
This invention also includes a display device utilizing the organic electroluminescence device described above for a pixel.
That is, a display device according to this invention basically has a constitution in which scanning lines for selecting pixels and data lines for providing luminance information for driving the pixels are arranged in a matrix, each of the pixels comprises an organic electroluminescence device that emits light in accordance with the amount of current supplied, a first active element controlled by the scanning lines and having a function of writing the luminance information given from the data lines to the pixels and, a second active element having a function of controlling the amount of current supplied to the organic electroluminescence device in accordance with the written luminance information, the luminance information is written to each of the pixels by applying electric signals in accordance with the luminance information to the data lines in a state where the scanning line is selected, the luminance information written in each of the pixels is held in each of the pixels even after the scanning line becomes no more selected, and the organic electroluminescence device in each of the pixels can maintain the light emission at a luminance in accordance with the luminance information.
The organic electroluminescence device comprises an anode, a cathode and an organic layer put between them and the organic layer contains an organic light emitting layer that emits light by re-combination of holes supplied from the anode and electrons supplied from the cathode.
The anode contains a metal belonging to the group V or group VI of the periodical table to at least a portion in contact with the organic layer.
The metal is preferably selected from chromium, molybdenum, tungsten, tantalum and niobium.
Further, the metal has a work function of 4.8 eV or lower.
Further, the cathode has a reflectance of 40% or higher. The anode is light reflecting and the cathode is light permeable in which emission light is mainly emitted from the side of the cathode.
Each of the pixels is accumulated and formed on the substrate, and the organic electroluminescence device contained in each of the pixels comprises the cathode, the organic layer and the anode laminated in this order from above to the substrate.
According to this invention, the anode of the organic electroluminescence device comprises a metal belonging to the group V or group VI of the periodical table. The metal can include high melting metals such as chromium, molybdenum, tungsten, tantalum and niobium. Such metals have a work function of 4.8 eV or lower, for example, chromium having 4.5 eV and tungsten having 0.6 eV. Further, the reflectance is 40% or higher.
Metals having somewhat higher work function of 4.8 eV or higher (Au, Pt, Ni, Pd, etc.) have been adopted for the anode so far in view of the requirement of supplying holes. In this invention, metals belonging to the group V or the group VI having lower work function (Cr, Mo, W, Ta, Nb, etc.) are used. It has been confirmed that even the metals belonging to the group V or the group VI can effectively supply the holes. Rather, chromium (Cr) and the like have less defects and excellent fabricability compared with gold (Au) and the like, as well as they are excellent as a whole as a material for the anode of the organic electroluminescence device.